1. Field of Invention
This invention relates in general to the checking and testing of semiconductor products using a charged particle microscope, and more particularly, to a method of forming precisely cross-sectioned electron-transparent samples which can be checked and tested using a charged particle microscope.
2. Description of Related Art
Cross-sectional side views of wafer chips are useful when investigating ultra large-scale integrated circuit components. Generally, sectioned samples are observed using electron or ion beam microscopes (also known as charged particle microscopes). For example, the most commonly used microscopes used for this task are scanning electron microscopes (SEM), transmission electron microscopes (TEM), scanning Auger microprobes (SAM) and focused ion beam (FIB) microscopes. When using these microscopes, the surface of the sample is bombarded with electrons or charged particles to create an observational image. Theoretically, the thinner the surface thickness of the sample, the better will be the resolution of the image viewed through the microscope.
A conventional method of preparing samples for observation using a charged particle microscope includes cleaving the wafer to form a wafer chip containing the feature to be investigated. Both faces of the wafer chip are polished using a chemical/mechanical polishing procedure until the targeted region is reached. This is followed by a lapping and a subsequent ion milling treatment to form a sample having a thickness of about 1000 .ANG. or less.
One disadvantage of the aforementioned method is that only one specific location in each sample is targeted. Therefore, this known method is not suitable when a large number of viewing sites coexist in the same wafer chip. Furthermore, the quality of the sample is difficult to control during the chemical/mechanical polishing procedure, so that the yield rate is low.
Another conventional method of preparing samples for observation using a charged particle microscope is shown in FIGS. 1A through 1E. Referring to FIG. 1A, a cleft and chemical/mechanical polished wafer chip 14 is mounted onto a copper grid 12. Next, and referring to FIG. 1B, a layer of metal 16 is deposited over the desired viewing site using an FIB microscope. Thereafter, and as shown in FIG. 1C, an H-shaped metallic mask 18 is defined using a high current FIB milling operation. Next, a low current FIB milling operation is used to mill the wafer surface, until a thin viewing surface 19 having a thickness of about 1000 .ANG. or less is obtained. The sample is now ready for observation under an electron microscope.
Referring to FIGS. 1D and 1E, when a wafer chip 14 has a plurality of desired viewing sites beneath its surface, an FIB microscope can be repeatedly used to deposit a plurality of metallic layers above the respective desired viewing sites. Then, the metallic layers can be similarly defined using FIB milling operations to form a plurality of H-shaped metallic masks 18'. Thereafter, FIB milling operations are again performed to further mill the wafer chip 14 in a plurality of locations, thereby forming a plurality of thin viewing surfaces 20 for observation with the electron microscope.
A major disadvantage of this method is that each milling operation can only form a viewing surface having a single desired thickness. Moreover, although the viewing surfaces are precisely obtained, each FIB milling operation is time consuming. For example, during the forming of viewing surfaces 20, the FIB milling operation must mill the wafer chip to a desired depth, and the viewing surfaces to the desired thickness. Thus, it may be too time consuming to completely prepare the sample when there are a plurality of viewing sites in the wafer chip.